PHILADELPHIA -- All-Star guard Isaiah Thomas has often provided the Boston Celtics with a finishing kick this season.

Thomas, who leads the NBA in fourth-quarter scoring, sat out Sunday's game against the Philadelphia 76ers with a sore right knee. The Sixers, as a result, were the ones who ran through the tape.

Dario Saric scored 23 points and Robert Covington nailed a go-ahead 3-pointer with 3:41 left to lift Philadelphia to a 105-99 victory.

"Finally, we beat them," said Saric, whose team had lost three games to Boston this season by eight points or fewer. "We deserved it, and the better team won today."

Boston had beaten the Sixers 10 straight times entering Sunday's game. Philadelphia, however, limited the Celtics to 25-percent shooting while outscoring them 29-20 in the fourth quarter -- 14-6 in the final 3:41.

"That's part of the growth of this team," said Covington, who finished with 16 points and eight rebounds. "Coach (Brett Brown) told us that you have to learn to figure out how to win games on our own. That's what we did this game."

Thomas, who is expected to return Monday night against Washington, is averaging 10.1 points a game in the fourth quarter. Without him Boston struggled down the stretch. Al Horford scored a season-high 27 points but missed all three of his shots in a scoreless fourth quarter, and while Jae Crowder collected eight of his 15 in the final period, he missed five of seven shots.

"They're going to play bully ball (without Thomas)," Brown said. "They're not just going to give it to Isaiah Thomas and walk down a game."

Crowder's two free throws with 4:55 left forged a 91-all tie, the last of the fourth quarter's four deadlocks. Saric then found Covington on the right wing, and he drilled his triple, putting Philadelphia ahead to stay.

"We were up all three games (earlier this season), and they just come like some kind of machine," Saric said. "Today we did not let them do that."

Saric acknowledged that Thomas' absence might have played a part.

"Big difference for them," he said, "but still we beat Boston."

Celtics coach Brad Stevens was not eager to make excuses, saying instead that the Celtics hurt themselves with their inefficient offense. That in turn created easy opportunities for the Sixers.

Boston, which saw a three-game winning streak end, shot just 40 percent from the floor, missed 23 of 29 3-point attempts and saw Philadelphia convert 18 turnovers into 22 points.

"If you don't play well for long periods of time, you're going to lose," Stevens said. "They were better than us the first quarter and a half, and better than us the last quarter and a half. If you give three quarters away in a game, you're going to get beat."

The Celtics started slowly, falling behind 33-25 early in the second quarter, but were awakened by reserve guard Terry Rozier. He was seemingly everywhere as Boston outscored Philadelphia 26-11 during an eight-minute stretch, scoring 10 of his 12 first-half points and grabbing four rebounds.

Bradley added six points in that spurt, which left the Celtics with a 51-44 lead. Their cushion was 51-46 at halftime, at which point Horford also had 12 points and Bradley 10.

Saric paced the Sixers with 14 points in the first half.

Horford packed 13 points into the first 5:22 of the third quarter, helping Boston seize a 69-56 lead, and the Celtics were still up 79-70 with 1:06 left in the quarter.

Philadelphia, however, scored the last six points of the quarter, on a tip-in by backup center Shawn Long, Long's fast-break dunk and, after a Boston turnover, a layup by Sergio Rodriguez with less than a second left.

"The mood of the gym changed (at that point)," Brown said, "and that really set the tone for a solid fourth period."

The Sixers surged in front on three occasions early in the fourth quarter, only to see Crowder answer each time.

NOTES: The Sixers' reserves outscored Boston's bench 43-34. ... Philadelphia coach Brett Brown, asked before the game about NBA teams' practice of resting key players, related it to his days as a San Antonio assistant, when coach Gregg Popovich regularly sat out key contributors during the regular season. Brown said it is a "combination of sports science, agents, clubs feeling the responsibility to keep your best players, especially, healthy" in the face of the months-long grind. "It's like water dripping on your forehead, this league," he said. "It just doesn't stop." ... Sixers G/F Justin Anderson sat out because of illness and C Jahlil Okafor left the game in the second quarter with a sore right knee. He did not return.